The present invention relates to image processing and more particularly to an image processing method and apparatus for converting an image of poor resolution to an image of improved resolution.
In the conventional Group 3 facsimile apparatus, two different resolution mode of images, a standard resolution mode and a fine resolution mode, are used for reading images, wherein the fine resolution mode provides a resolution of image in a vertical scanning direction set twice as high as that of the image of the standard resolution mode.
In such a case, a picture element constituting the image of the standard resolution mode shown in FIG. 1A has a size set such that a size LM1 in the horizontal scanning direction is 1/8 mm and that a size LS1 in the vertical scanning direction is 1/3.85 mm, while a picture element constituting the image of the fine resolution mode shown in FIG. 1B has a size LS2 in the vertical scanning direction which is set to 1/7.7 mm as shown in FIGS. 1A and 1B. Note that the size of the picture element of FIG. 1B in the horizontal direction is LM1 and this size is identical to that of the picture element of FIG. 1A.
Naturally, the fine resolution mode provides an improved resolution when the image is read in the fine resolution mode and reproduced also in the fine resolution mode because of the reduced size of picture element.
Recently, facsimile apparatuses having a capability of reading and recording an image with a resolution increased twice as large as the resolution in the fine resolution mode in both the horizontal scanning direction and the vertical scanning direction are developed and used. Such a mode of resolution will be referred to hereinafter as a super-fine resolution mode.
FIG. 1C shows a picture element used in the super-fine resolution mode. Referring to FIG. 1C, it will be understood that the picture element has a size LM2 in the horizontal scanning direction which is set to 1/16 mm and a size LS3 in the vertical scanning direction which is set to 1/15.4 mm. Thus a further improved quality of picture is achieved with respect to the resolution when the image is read in the super-fine resolution mode and reproduced also in the super-fine resolution mode.
When a facsimile apparatus having a capability of reading and recording an image with the super-fine resolution mode is used to receive a facsimile transmission from another facsimile apparatus which does not have the capability of the super-fine resolution mode but the capability of only the fine resolution mode, the transmitted image has of course the resolution of the usual fine resolution mode and thus, the reproduced image has a size which is demagnified to one half of the original image with respect to the horizontal scanning direction as well as with respect to the vertical scanning direction, when the transmitted image is reproduced as it is.
In order to avoid such an inconvenience of unwanted demagnification of the reproduced image, the facsimile apparatus having the super-fine resolution mode capability usually has an additional capability to magnify the size of the reproduced image twice when reproducing a facsimile image transmitted in accordance with the fine resolution mode.
In such a conventional facsimile apparatus, although an image with the quality of the fine resolution mode is obtained, this image is of course inferior, with respect to the quality, to the images of super-fine resolution mode which the facsimile apparatus is potentially capable of providing. As most of the facsimile apparatuses currently used do not have the capability of super-fine resolution mode because of the high price, the facsimile apparatus having the super-fine resolution mode receives facsimile transmission in most of the time in the standard mode or in the high resolution mode. Thus, there are cases that the user of the facsimile apparatus having the super-fine resolution mode capability feels dissatisfaction about the performance of the facsimile apparatus.
In order to reconcile with the demand by the user to achieve a further improved quality of images even when the facsimile apparatus receives a facsimile transmission with the fine resolution mode, the facsimile apparatus having the super-fine resolution mode conventionally has a capability of so-called smoothing process.
In the smoothing process used for this purpose, an obliquely elongating boundary region defining a boundary between a region consisting of white picture elements and a region consisting of black picture elements and extending with an angle of 45 degrees with respect to the horizontal scanning direction, is processed such that the resolution in this region is converted to that of the super-fine resolution mode.
According to the statistical analysis of images, it is known that the lines extending with an angle of 45 degrees (FIG. 2) with respect to the horizontal scanning direction assumes a proportion of about 50% of the lines in the images when the lines parallel to the horizontal and vertical scanning directions are eliminated. Thus, a substantial improvement on the quality of the reproduced image is expected by such a limited effort.
FIG. 3A shows an example of a field of a fine resolution mode image comprising nine picture elements a, b, c, . . . , g and h therein. Such a field appears at the boundary of the image such that a central picture element x coincides to the boundary. Thus, when the boundary is linear and extending from the lower left to the upper right with the black picture elements in the lower right half and the white picture elements in the upper left half of the field as shown in FIG. 4A, the picture elements c, e, f, g and h become black while picture elements a, b and d become white.
In such a field, the picture element x at the center is converted to the super-fine resolution mode by dividing the picture elements to four small picture elements x.sub.00, x.sub.01, x.sub.10, and x.sub.11 as shown in FIG. 3B, wherein the small picture element x.sub.00 is set to white while the other small picture elements x.sub.01, x.sub.10, x.sub.11 are set to black as illustrated in FIG. 4A.
When the central picture element x is white as shown in FIG. 4B together with additional white picture elements at the picture elements c, e, f, g and h and further with black picture elements at the picture elements a, b and d, the smoothing process is performed such that the small picture element x.sub.00 becomes black while the other small picture elements x.sub.01, x.sub.10 and x.sub.11 remain all white.
When the boundary of image is not forming the angle of 45 degrees with respect to the horizontal scanning direction in correspondence to the line not forming the angle of 45 degrees with respect to the horizontal scanning direction, the entire small picture elements x.sub.00 -x.sub.11 are set identical to the central picture element x of the field of FIG. 3A.
By moving the field of FIG. 3A along the boundary of the image, the stepwise boundary image in the fine resolution mode shown in FIG. 5B is converted to the super-fine resolution mode shown in FIG. 5B.
In such a prior art facsimile apparatus performing the smoothing process, however, there is a problem in that when a facsimile transmission comes in with the standard resolution mode, the reproduced image has an unwantedly steep slope at where the smoothing process is applied. For example, in a case of FIG. 6A where an image containing an oblique line having an angle of 45 degrees with respect to the horizontal scanning direction comes in in the standard resolution mode, the central picture element x is subjected to smoothing by being divided into the foregoing four small picture elements x.sub.00 -x.sub.11 As the picture element x as well as other picture elements a-h in FIG. 6A are the picture elements of the standard resolution mode, each of the picture elements therein has a horizontal size of LM1 and a vertical size of LS1 as defined in FIG. 1A. Thus, each of the small picture elements x.sub.00 -x.sub.11 has a shape expanded twice in the vertical direction as compared to the corresponding picture elements in FIG. 3B and thereby the slope corresponding to the picture element X after the smoothing processing inevitably becomes steep as shown in FIG. 7A in the part surrounded by a broken line. Such a steep part appears in each of the corners X', X", . . . in FIG. 7A and provides an unnatural feeling of the image. In other words, there is a problem in the conventional smoothing process that, when this smoothing is applied to the images of the standard resolution mode, the resultant image is not improved but degraded. Further, such an image is not suited for further conversion to the super-fine resolution mode image as each of the small picture elements is elongated in the vertical scanning direction and the pattern as shown in FIG. 4A or FIG. 4B comprising square picture elements is not obtained after the smoothing.
FIG. 6B and 7B show the case where the black region and the white region are reversed as compared to the case of FIGS. 6A and 6B. In this case, too, a similar undesirable effect is obtained as shown.